Communication pubs.acs.org/crystal
Supramolecular Synthon Transferability and Gelation by Diprimary Ammonium Monocarboxylate Salts Uttam Kumar Das,† Vedavati G. Puranik,‡ and Parthasarathi Dastidar*,† †
Department of Organic Chemistry, Indian Association for the Cultivation of Science, 2A&2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, West Bengal, India ‡ Centre for Materials Characterization, National Chemical Laboratories, Dr. Homi Bhaba Road, Pune 400008, India S Supporting Information *
ABSTRACT: Earlier studies revealed that primary ammonium dicarboxylate (PAD) salts possessed gelling ability, and many such salts displayed a 1D columnar hydrogen bonded network (observed in primary ammonium monocarboxylate (PAM) salts) on either side of the dicarboxylate end of the anion. In the present study, a new series of diprimary ammonium monocarboxyate (DPAM) salts have been prepared with the aim of achieving supramolecular synthon transferability (the same 1D columnar hydrogen PAM bonded network on either side of the diammonium cation) in these salts. Single crystal Xray diffraction studies revealed that, in 47% of the DPAM salts, such supramolecular synthon transferability indeed took place. Some of the DPAM salts also showed gelation ability. The gels were characterized by DSC, rheology, electron microscopy, and atomic force microscopy. Structure property correlation using single crystal and powder X-ray diffraction data on a selected gel was also attempted.
L
ow molecular weight gelators (LMWGs)1 are small molecules (molecular weight < 3000) that are capable of solidifying (gelling) organic solvents (organogels), pure water, or aqueous solvents (hydrogels) and are in high demand because of their potential applications.2 Unfortunately, most of the gelators discovered are serendipitously found, and subsequent modification of the parent gelator molecules led to the development of a new generation of gelators. Designing gelators ab initio is a challenging task, as the molecular level understanding of the gel formation mechanism is not fully deciphered. Various studies indicate that the LMWG molecules form self-assembled fibrilar networks (SAFINs)3 driven by different noncovalent interactions, such as hydrogen bonding, π−π stacking, hydrophobic/hydrophilic, and van der Waals interactions; solvent molecules are then immobilized within the SAFINs, resulting in gel formation. Key to any design strategy is to have a detailed understanding of the structure of the material under study. Thus, determining the structure of SAFIN is important in designing a gelator molecule. Since SAFINs are too tiny to carry out single crystal X-ray diffraction (SXRD) and powder X-ray diffraction (PXRD) is not yet a routine methodology, an alternative indirect approach is proposed by Weiss and co-workers4 wherein experimental and simulated PXRD patterns are compared in order to determine the structure of SAFINs. Such a school of thought ultimately led to the working hypothesis that indicates that 1D and 2D networks help promote SAFIN formation whereas a 3D network is not as good for such a task.5 The fact that such a hypothesis was based © 2012 American Chemical Society
on a logical foundation was explicitly demonstrated by our group6 nearly a decade ago, and since then, we have been engaged in exploiting the supramolecular synthon concept7−an essential tool in crystal engineering8−in designing LMWGs.9 We have thus far developed various supramolecular synthons, such as secondary ammonium monocarboxylate (SAM),10 secondary ammonium dicarboxylate (SAD),11 primary ammonium monocarboxyate (PAM),12 and primary ammonium dicarboxyate (PAD),13 in discovering a new series of gelators (Scheme 1). Among these synthons, the PAD synthon displayed remarkable properties. Most often it is a 2D synthon by virtue of the bifunctionality of the anionic part (carboxyate) of the ion pair. We have shown that the 2D PAD synthon can be folded into a 1D nanotubular construct driven by alkyl−alkyl hydrophobic interactions induced hydrogen bond isomerism.13 A close look at the detail of the hydrogen bonding interactions in the 2D PAD synthon reveals the presence of a columnar PAM synthon (synthon W or X, Scheme 1) at each carboxylate end of the dianion. Supramolecular synthon transferability is an important issue in crystal engineering, as it has farfetched implication in designing solids with desired structures and properties.14 In an attempt to study the supramolecular synthon transferability of the 2D PAD synthon, we consider studying the reverse of the PAD salt wherein a α,ω-diamine (instead of α,ω-dicarboxylic Received: August 28, 2012 Revised: October 11, 2012 Published: October 12, 2012 5864
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Scheme 1. Various Supramolecular Synthons Studied by Our Group
of the structures display the 2D network as expected; synthon transferability has been observed in 7 salts. Interestingly, some of the salts are capable of displaying organo- and hydrogelation properties (Supporting Information). The gels are characterized by DSC, rheology, SEM, AFM, etc. Structure−property (gelation) correlation based on SXRD and PXRD data has also been reported. The DPAM salts were prepared by reacting the corresponding diamines and the acids in MeOH at room temperature with near quantitative yield. Crystals suitable for single crystal X-ray diffractions were grown following the slow evaporation method from suitable solvent systems (Supporting Information). SXRD data revealed that, out of 15 salts, 7 salts displayed the 2D DPAM synthon; none of the B3A(X) salt displayed the 2D DPAM synthon; 3 of the B4A(X) salts [B4A(3-NO2), B4Q, B4An] showed PAM synthon X at each ammonium end whereas four salts [B4A(4-Cl), B4A(2-Br), B4A(4-Br), and B4A(4-NO2)] displayed PAM synthon W in the diammonium sites. It may be noted that the diammonium cation B4 in the salts displaying the 2D DPAM synthon adopted an extended allstaggered conformation which presumably helps the ion pairs to self-assemble in a 2D network. However, in salt B3A(4-Me), the diammonium cation B3 adopted a “U” shaped conformation that resulted in 1D PAM synthon X (Figure 1). The rest of the salts [B3A(2-Cl), B3A(3-Cl), B3A(3-Br), B3A(3-NO2), B3A(4-NO2), and B4A(2-NO2)] displayed a 2D network having various hydrogen bonding connectivities; this appears to be due to the various conformations (all staggered, staggered-gauche, etc.) adopted by the diammonium cations in these salts; in the case of B4A(4-Me), lattice occluded water participated in hydrogen bonding, thereby displaying different hydrogen bonding connectivity (Supporting Information). Thus, it is clear that interchanging of the dicarboxylic acid/ primary amine to monocaroxylic acid/diprimaryamine did not
acid as in the case of PAD salt) is reacted with a monocarboxylic acid in 1:2 molar ratio. It is expected that such an interchange should not drastically change the overall supramolecular network of the 2D PAD synthon. Since it is derived from a primary diamine and a monocarboxylic acid, we hereafter designate this new synthon as diprimary ammonium monocarboxyate (DPAM). In this article, we report the single crystal structures of 15 salts out of 26 DPAM salts that we prepared (Scheme 2); most Scheme 2. List of 26 DPAM Salts Reported in This Paper
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participation of the noncovalent interactions such as hydrogen bonding in forming the gel network.
Figure 2. Tgel vs [gelator] plot (DCB-1,2-dichlorobenzene, MS-methyl salicylate, NB-nitrobenzene).
Differential scanning calorimetry (DSC) was performed on a selected gel [a 6.0 wt % w/v methylsalicylate gel of B3A(4-Me) in order to study its thermoreversibility in a quantitative fashion. Clearly, two peaks at ∼100 °C and ∼58 °C in endothermic and exothermic cycles, respectively, could be seen; while the former represents the gel−sol dissociation temperature (Tgel), the later is the sol−gel transition temperature (Figure 3).
Figure 1. Synthons observed in the crystal structures of some of the DPAM salts: (A) 2D PAD synthon having PAM “X” observed in B4A(3-NO2), B4Q, B4An; (B) 2D PAD synthon having PAM “W” observed in B4A(4-Cl), B4A(2-Br), B4A(4-Br), and B4A(4-NO2); (C) 1D PAM synthon “X” observed in B3A(4-Me).
influence the detail of the 2D hydrogen bonding network (HBN) in 7 DPAM salts, thereby confirming the synthon transferability (Scheme 3). Scheme 3. Transformation of 2D PAD Synthon into 2D DPAM Synthon
Figure 3. DSC trace of the ∼6 wt % methyl salicylate gel of B3A(4Me).
The dynamic rheology for two selected gel samples [B4A(4Me) and B4A(3-NO2)] has been carried out to study the viscoelastic behavior of the corresponding gels. Here, the storage or elastic modulus, G′, and the viscous modulus, G″, were plotted as a function of angular frequency, ω, at the constant strain 0.1%. It is observed (Figure 4) that the storage modulus, G′, is invariable as a function of angular frequency over a considerable time period and the magnitude of G′ (95.7 and 56.1 kPa for B4A(4-Me) and B4A(3-NO2), respectively) is much larger than that of G″ (19 and 25 kPa for B4A(4-Me) and B4A(3-NO2), respectively), which supported strongly the viscoelastic behavior of these gels. To study the morphology of the gel network of the xerogels, we have performed SEM, TEM, and AFM on the 1,2-
Our previous report on the gelation ability of the PAD salts and the synthon transferability in the present case encouraged us to undertake detailed gelation studies of the DPAM salts reported herein. Gelation tests were carried out in 15 selected solvents that include both the polar and the nonpolar solvents (Supporting Information). Interestingly, 5 salts displayed gelation ability. The minimum gelator concentration (MGC) and the gel−sol dissociation temperature (Tgel) are within the range 2.2−4.0 wt %, w/v, and 76−128 °C, respectively indicating that the gelators were quite efficient and the gels were remarkably stable. Tgel vs [gelator] plots of 5 selected gels displayed a steady increase in Tgel with the increase in [gelator] (Figure 2). These results were attributed to the active 5866
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Figure 4. Frequency sweep rheological traces of two selected gels.
Figure 5. SEM of (A) B3A(3-Cl); (B) B4A(4-Me); (C) B3A(3-Me); (D) B3A(3-Me); and (E) B3A(4-Me). TEM of (F) B3A(3-Cl). AFM of (G) and (H) B3A(3-Cl).
To determine the structure of the gel fiber in the xerogel state, we have undertaken detailed X-ray diffraction experiments on a selected salt, namely B4A(3-NO2). Figure 6 depicts the comparison plot of PXRD patterns15 of B4A(3-NO2) under various conditions; nearly superimposable PXRD patterns clearly establish that the structure of the gel network in the xerogel (4 wt % in water) is identical with that of the SXRD structure of the salt. Thus, a library of 26 DPAM salts has been generated, and single crystal structures of 15 such salts have been determined. SXRD data established that the 2D PAD synthon is indeed transferable to the DPAM salt system in ∼47% of the salts studied herein. The conformational flexibility of the diammo-
dichlorobenzene xerogel of some selected gelator salts. The samples for microscopic experiments were prepared by dropcasting a much diluted solution on the SEM stub/TEM grid in order to get a clear view of the morphology (Supporting Information). Unlike highly entangled 1D fibers usually observed in the SEM of innumerous gels, aggregates of spherical particles could be seen in most of the cases except in the case of B3A(3-Cl), wherein a platelike morphology was observed. It is interesting to note that, in the case of B4A(4Me), highly aligned aggregates of particles could be seen. High resolution microscopic data such as TEM/AFM also confirm the presence of a platelike and spherical morphology in B3A(3Cl) and B3A(4-Me), respectively (Figure 5). 5867
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(6) Trivedi, D. R.; Ballabh, A.; Dastidar, P.; Ganguly, B. Chem.Eur. J. 2004, 10, 5311. (7) Desiraju, G. R. Angew. Chem., Int. Ed. 1995, 34, 2311. (8) Desiraju, G. R.; Vittal, J. J.; Ramanan, A. Crystal Engineering A Text Book; IISc Press, World Scientific: India, 2011. (9) Dastidar, P. Chem. Soc. Rev. 2008, 37, 2699. (10) Trivedi, D. R.; Ballabh, A.; Dastidar, P. J. Mater. Chem. 2005, 15, 2606. (11) Sahoo, P.; Kumar, D. K.; Trivedi, D. R.; Dastidar, P. Tetrahedron Lett. 2008, 49, 3052. (12) Das, U. K.; Trivedi, D. R.; Adarsh, N. N.; Dastidar, P. J. Org. Chem. 2009, 74 (18), 7111. (13) Ballabh, A.; Trivedi, D. R.; Dastidar, P. Org. Lett. 2006, 8 (7), 1271. (14) Aakeroy, C. B.; Scott, B. M. T.; Smith, M. M.; Urbina, J. F.; Desper, J. Inorg. Chem. 2009, 48 (9), 4052. (15) Piepenbrock, M-O. M.; Lloyd, G. O.; Clarke, N.; Steed, J. W. Chem. Commun. 2008, 2644.
Figure 6. PXRD plots of B4A(3-NO2) under various conditions.
nium alkane spacer plays a crucial role in shaping up the overall hydrogen bonding network in the resulting salts. Thus, 7 DPAM salts failed to show synthon transferability, mainly because of the various conformations adopted by the diamonium cation. The effect of the conformational flexibility of the diamonium backbone is remarkable in the salt B3A(4Me); herein, the diammonium cation adopted a “U” conformation, which essentially prevented the formation of a 2D HBN leading 1D PAM synthon. The fact that 5 salts out the 26 DPAM salts reported herein displayed reasonable gelation ability goes well with the supramolecular synthon applicability in designing LMWGs.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental data, single crystal structure description, molecular plot, hydrogen bonding parameter, crystallographic parameter, CIF data, and gelation data table. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]; parthod123@rediffmail.com. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS P.D. and U.K.D. thank CSIR, New Delhi, for financial support and a SRF fellowship, respectively. Single crystal X-ray diffraction data were collected at the DST-funded National Single Crystal Diffractometer Facility at the Department of Inorganic Chemistry, IACS, and National Chemical Laboratory, Pune.
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REFERENCES
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